The present invention relates to an oscillating driver circuit with power factor correction, suitable, for instance, for driving a load at a near unity power factor. For example, the present invention is particularly suited for use as an electronic ballast for driving fluorescent lamps or other gas discharge illumination devices.
Electronic ballast circuits have recently undergone a revolution in sophistication from the early bipolar designs of ten years ago. This has been brought about partly by the advent of power MOSFET switches which have inherent advantages in efficiency and also by incentives and utility rebate programs sponsored by domestic and foreign governments. New I.E.C. requirements have also spurred the design of high power factor ballasts and are starting to impose further restrictions on harmonic content of current waveforms for equipment operated from AC mains.
Before the burgeoning interest in these new ballast designs made possible by the power semiconductor industry, coil and core electromagnetic fluorescent ballasts were manufactured in large quantities by a few key suppliers.
Today, there are many electronic companies that are "in the ballast business" producing electronic ballasts.
Most electronic ballasts use two power switches in a totem pole (half-bridge) arrangement functioning as an inverter which derives power from a dc bus. The dc bus voltage is provided by a rectifier operating from the ac mains supply. The illumination tube circuits consist of L-C series resonant circuits with the lamp(s) connected across one of the reactances. FIG. 1 shows a basic prior art electronic ballast.
In the circuit of FIG. 1, the electronic switches 10 and 20 are power MOSFETs driven to conduct alternately by windings on a current transformer 12. The primary 12A of transformer 12 is driven by the oscillating current in the lamp circuit 14 and operates at the resonant frequency of inductor L and the capacitor C at the input of the transformer winding 12A. Capacitor 18 is a coupling capacitor coupling the lamp circuit 14 to the electronic ballast and typically has a much higher capacitance value than capacitor C. Capacitor 18 determines the resonant frequency once the lamp 14 strikes, because the lamp voltage will effectively short capacitor C once the lamp lights.
Without a trigger device, the inverter circuit of FIG. 1 is not self starting. Accordingly, it must be pulsed by a DIAC 16 or other trigger device connected to the gate of the lower MOSFET 20. The trigger device 16 is triggered by charge on a capacitor 19 in series with resistor 21 coupled to dc bus D.
After the initial turn-on of the lower switch 20, oscillation is sustained and a high frequency square wave (30-80 kHz) excites the L-C resonant lamp circuit 14. The sinusoidal voltage across capacitor C is magnified by the Q at resonance and develops sufficient amplitude to strike the lamp 14A, which then provides flicker-free illumination. The frequency of oscillation is thereafter determined by inductor L and capacitor 18.
This basic circuit has been the standard for electronic ballasts for a number of years but has the following inherent shortcomings:
1) It is not self starting without a triggering device;
2) It has poor switch times;
3) It is labor intensive to manufacture because of the torroidal current transformer 12;
4) It is not amenable to dimming; and
5) It is expensive to manufacture in large quantities because of the current transformer.
Another recent development has been the introduction of integrated circuit chips for driving power MOSFETS and IGBTs in inverter circuits such as used in electronic ballasts. These integrated circuit chips, known as MOS Gate Drivers (MGDs), offer significant cost, weight and space savings over driver circuits employing current transformers.
An example of an MGD is the IR2155 device available form International Rectifier Corporation. This device provides a self oscillating function which has been found to be particularly suited for use in inverter circuits, e.g., electronic lamp ballast circuits.